1. Field of the Invention
The present invention relates to an autofrettage process and, more particularly, relates to the autofrettage of thick-walled cylinders through the cyclical application of high pressure to the internal bore of the cylinder.
In the design of thick-walled cylinders which are utilized as compressor cylinders, pump cylinders, high pressure tubing, process reactors and vessels, nuclear reactors, isostatic vessels, or gun barrels and the like, various fatigue and fracture characteristics must be taken into consideration when selecting the material and in choosing the various dimensions. Cylinders of this type which are generally subjected to sudden and frequently dramatic pressure fluctuations, such as the pressure generated in a gun barrel upon the firing of the weapon, pressure reversals in pump cylinders or in process reactors employing high-pressure piping, are prone to premature failure as the result of material fatigue or embrittlement produced by the internal pressures to which the bores of the cylinders are exposed. Usually, the material employed for such cylinders is steel; normally an alloy steel having superior strength and fatigue properties.
In order to alleviate the problems encountered in subjecting thick-walled cylinders to alternating or fluctuating, internally acting high pressures, there has been developed a process for enhancing the strength of thick-walled cylinders which are subjected to repeated internal pressures. This process is commonly referred to as "autofrettage" and involves the application of such interior pressure to the bore of the cylinder so as to plastically deform at least the inner layers of the cylinder material beyond the elastic limit or yield strength of the material and to thereby generate "negative" or residual tangential compressive stresses at the cylinder bore. These residual stresses will counteract the destructive effects of the internal cyclical or intermittent high operating pressures to which the cylinder is subjected so as to appreciably extend the service life of the cylinder. This plastic deformation of the inner bore of the cylinder beyond the elastic limit of the material so as to increase the elastic limit is a function of the strength of the material, in essence, the type of steel or alloy utilized, and of the ratio of the outer diameter to the inner diameter of the cylinder. In effect, the basic advantage obtained through the autofrettaging of thick-walled cylinders which are subjected to cyclical, intermittent or fluctuating internal high operating pressures lies in that the flexibility of the cylinders in response to these operating pressures is increased to a considerable extent to thereby impart a longer service life to the cylinders.
2. Discussion of the Prior Art
Processes for the autofrettage of thick-walled cylinders have been known for a lengthy period of time and, basically, fall into one of two major categories, as follows:
(a) The insertion of an oversized mandril or ram into the bore of the cylinder so as to exert radial pressures through forced expansion or stretching of the bore and, resultingly, of the cylinder. This rather crude mechanical process, however, frequently fails to provide the desired results in the necessary formation of residual tangential compressive stresses at the bore of the cylinder and, at times, actually leads to the destruction of the cylinder.
(b) The generation of a high pressure within the cylinder beyond the elastic limit of the material so as to cause the plastic deformation of the layers of material extending radially outwardly of the bore either partially or entirely through the wall thickness of the cylinder. For presently employed high-strength alloy steels this may necessitate an autofrettage pressure of up to about 200,000 psi. This extremely high pressure is generally disadvantageous in that it does not always produce the required residual tangential compressive stresses in the bores of the cylinders and, additionally, deforms the cylinder bore to an extent wherein the cylinder is no longer employable in its intended use, for example, in connection with its utilization as a gun barrel or piston cylinder in which the size of the cylinder bore must be maintained within narrowly prescribed bounds. Moreover, the extremely high autofrettaging pressure requires the use of large, complex and expensive pressure-generating and applicating equipment.
Thus A. E. Guy U.S. Pat. No. 1,329,762 discloses an installation for applying expanding pressure to a material or filler within the bore of a thick-walled cylinder in order to effect an autofrettage process in a single-step operation. This patent disclosure is not capable of carrying out a precise cylclical application of autofrettage pressure analgous to the present invention in which lower pressures will afford the desired cylinder stressing in a simplified and inexpensive manner.
Although F. C. Langenberg U.S. Pat. Nos. 1,553,824 and 1,553,825 illustrate and disclose multiple-step pressure applications, with the possible interposition of an annealing for softening the cylinder material, there is no mention of the only partial plastic deformation of the inner material layers of the cylinder as in the present instance, but rather the total controlled expansion of the cylinder. In addition thereto, the pressure medium employed in the Langenberg U.S. patents is water, which is not suitable at the cyclical pressures employed for the autofrettaging process of the instant invention.